1. Field of the Invention
The present invention relates to a shuttle mechanism of a printer apparatus where a print head reciprocates so as to carry out printing.
2. Description of Related Art
FIG. 1 is a schematic appearance of a dot line printer apparatus. In FIG. 1, numeral 7 designates a paper feed unit, numeral 8 designates a print mechanism section containing a print shuttle unit with a print head mounted thereon, and numeral 9 designates a ribbon unit equipped with an ink ribbon. While the print head is making linear reciprocating motions, it strikes the ink ribbon of the ribbon unit 9 and carries out printing to a paper 4 sent to the inside of the printer by the paper feed unit 7. When the printing operation of one line is completed, the paper 4 is sent in the vertical direction against the direction of reciprocating motion and the continuous printing is carried out.
As described above, in a line printer apparatus, since a print shuttle unit with a print head mounted thereon makes reciprocating motions at high speed, such a structure as the whole apparatus is not vibrated due to the reciprocating motion is essential. Consequently, various structures to eliminate the vibration have been proposed in the prior arts.
According to Japanese Patent Application Laid-Open No. 61-169257 (1986), published is a line printer apparatus where a linear motor for reciprocating motions of a print shuttle unit with a print head thereon is constructed by a magnetic coil fixed to the lower side of the print shuttle unit and a movable type magnetic circuit section driving the magnetic coil. The magnetic circuit section of the linear motor is moved in the reverse direction against the print shuttle unit, and thereby the magnetic circuit section acts as a balancer of the print shuttle unit and the vibration is eliminated.
In the above-mentioned constitution, however, in order that the reciprocating motions in reverse phase of the print shuttle unit and the magnetic circuit section are maintained for a long period, both must be connected by a wire and the control of the reciprocating motions is not easy. On the other hand, a line printer apparatus is well known, where a balance shuttle unit made in nearly the same weight as a print shuttle unit makes reciprocating motions interlocking with the reciprocating motions of the print shuttle unit and in parallel and in reverse direction to this.
In the conventional apparatus of such a construction, the reaction force produced in a base frame or the like attendant along with the reciprocating motions of the print shuttle unit is offset and generation of the vibration is suppressed.
In the above-mentioned construction, however, when the print shuttle unit and the balance shuttle unit are moved in parallel in the reverse directions, rotational moment is generated due to the couple while both are moving. Thereby the rotational vibration is generated in the whole apparatus and the print quality may become bad due to the vibration of the printing paper.
In order to eliminate such disadvantages, an improved example to this is proposed in Japan Patent Application Laid-Open No. 6-040108 (1994). FIG. 2 is a perspective view of a shuttle mechanism in this improved example, and FIG. 3 is a sectional side elevation view, of the same. In FIG. 3, a white arrow indicates the direction of the gravity.
A print shuttle frame 12 with a print head 11 mounted thereon is slidably fitted to a shuttle shaft 2 arranged nearly at the center of the shuttle mechanism. The print shuttle frame 12 is supported by a roller 13 capable of traveling on a base frame 6 and the shuttle shaft 2. A plurality of coils 16 are arranged at a lower surface of a coil base plate 14, an iron plate, attached at the lower side of the print shuttle frame 12. A print shuttle unit 10 is composed of the print shuttle frame 12, the print head 11 and the coil base plate 14 mounted thereon and the coils 16, and can be moved along the shuttle shaft 2.
Opposed to the coils 16 arranged on the print shuttle unit 10, a plurality of permanent magnets 15 are arranged on a yoke 18, an iron plates fixed to the base frame 6 with a small gap between magnets 16 and the coils 16. A linear motor for driving the print shuttle unit 10 is constructed by the permanent magnets 15 and the coils 16. In the linear motor, current flows through the coils 16 within the magnetic field by the permanent magnets 15, and thereby thrust based on the Fleming's left-hand rule is generated in the coils 16 and the print shuttle unit 10 with the coils 16 mounted thereon is moved along the shuttle shaft 2. The current flowing through the coils 16 is controlled, and thereby the print shuttle unit 10 can be moved in reciprocating motion at high speed.
A balance shuttle frame 22 similar to the print shuttle frame 12 is slidably fitted to a shuttle shaft 3 arranged in parallel to the shuttle shaft 2. The balance shuttle frame 22 is supported by a roller 23 capable of traveling on the base frame 6 and the shuttle shaft 3. A weight 21 is mounted on the balance shuttle frame 22, and a plurality of coils similar to the coils 16 are arranged at a lower surface of a coil base plate 24 attached to the lower side of the balance shuttle frame 22. A pair of arms 29 projecting on both lateral sides of the balance frame 6 are coupled with the balance shuttle frame 22. The top end of the arm 29 gets beyond the installation position of the print shuttle unit 10 and reaches the opposite side, and a balance weight 30 is mounted on the top end. A roller 31 capable of traveling on the base frame 6 is attached to the balance weight 30. A balance shuttle unit 20 is composed of the balance shuttle frame 22, weight 21 and coil base plate 24 mounted on the balance shuttle frame 22, coils 26, arm 29 and balance weight 30 mounted on the top end of the arm 29, and can be moved along the shuttle shaft 3.
Opposed to the coils 26 arranged on the balance shuttle unit 20, a plurality of permanent magnets 25 similar to the permanent magnets 15 are arranged with a small gap between the magnets 25 and the coils 26 on a yoke 28 similar to the yoke 18.
A linear motor for driving the balance shuttle unit 20 is constructed by the permanent magnets 25 and coils 26. The linear motor for driving the balance shuttle unit 20, which is similar to that for driving the print shuttle unit 10, controls the current flowing through the coils 26, and thereby the balance shuttle unit 20 can be moved in reciprocating motion at high speed along the shuttle shaft 3.
The total weight of the balance shuttle unit 20 is nearly the same as that of the print shuttle unit 10. Also the weight distribution of the balance shuttle unit 20 is effected so that the traveling line of the center of gravity of the whole balance shuttle unit 20 while it is moving along the shuttle shaft 3 gets to nearly the same position as the traveling line of the center of gravity of the whole print shuttle weight 10 while it is moving along the shuttle shaft 2. In addition, numeral 5 in FIG. 3 designates a guide roller to feed the paper 4 into the shuttle mechanism (print mechanism section).
In the shuttle mechanism constituted as described above, when the print shuttle unit 10 is moved in reciprocating motions along the shuttle shaft 2, the balance shuttle unit 20, nearly the same weight as the print shuttle unit 10, is moved in reciprocating motions interlocking with the print shuttle unit 10 in the reverse direction at the same speed along the shuttle shaft 3. Consequently, the reaction force produced in the base frame 6 to the reciprocating motion of the print shuttle unit 10 is offset by the reciprocating motion of the balance shuttle unit 20. Also in this case, since the center of the gravity of the the balance shuttle unit 20 moves on the same line as the center of the gravity of the print shuttle unit 10 travels, the rotational moment is not generated due to the reciprocation motion of both. As a result, the vibration of the whole apparatus can be reduced.
In the above-mentioned construction, since, besides the balance shuttle frame 22, the arm 29 and the balance weight 30 makes the reciprocating motions interlocking with the print shuttle unit 10, a large mounting range is necessary for these members. Consequently, there is a problem that the required miniaturization of the apparatus is inhibited and the operability of the apparatus is deteriorated.
Also the shuttle mechanism shown in FIG. 2 and FIG. 3 realizes the suppression of the rotational moment produced due to the couple attendant on the reciprocating motion of the print shuttle unit 10 and the balance shuttle unit 20 by installing the balance weight 30 and making the traveling line of the center of the gravity of both shuttle units 10, 20 nearly the same. Consequently, mounting of the balance shuttle unit 20 for the balance weight 30 and the arm 29 mounting it must be carried out with good accuracy.
As the spaced difference between the print shuttle unit 10 and the balance shuttle unit 20 becomes longer, the balance weight 30 requires the heavier weight and the mounting design is subjected to the larger restriction. When the print shuttle unit 10 and the balance shuttle unit 20 are arranged closely and the positions of the center of the gravity of both shuttle units 10, 20 are nearly the same, the rotational moment as above described is not generated, and since the original balance operation of the balance shuttle unit 20 by the weight 21 only can be realized without the balance weight 30, a problem in mounting as above described is not produced.
Accordingly, it is essential to devise the mounting method which makes the positions of the center of gravity of both shuttle units 10, 20 as close as possible. However, in a method of mounting the balance shuttle unit 20 to the opposite side of the printing shuttle unit 10 against the paper traveling region interposed between both as seen in the above-mentioned shuttle mechanism, the reduction of the distance between both shuttle units 10, 20 has limitation. Because a space for the paper traveling path and an installation space of mechanism parts relating to printing such as a platen roll must be provided between both shuttle units 10, 20. Consequently, the mounting method which makes the positions of the center of gravity of both shuttle units 10, 20 as close as possible cannot be a reasonable mounting method.